The present invention relates to a through hole construction for process measuring devices transferring force or movement from the measuring element communicating with the process substance to a measuring mechanism outside the process substance.
Such measuring devices are in wide use in the measuring operations applied in various processes. For carrying out the measuring operation, the measuring devices are provided with a sensing element in communication with the process substance and effected by a force resulting from the process substance to be measured. The device is also provided with a lever-like transfer element connected with the sensing element and transferring the force acting on the sensing element using the torque about the transverse axis of the transfer element to a member performing the actual measuring operation and being located outside the process substance. The transfer member is led from the portion communicating with the process substance to the portion outside the process substance through a through hole. The through hole must in this case have a sealing member for separating the portion outside the process substance and at the same time the measuring mechanism from the process substance. Because the purpose is to measure the force, the construction at the through hole must be as elastic as possible in order to minimize the forces acting on the transfer member and resulting from the construction of the through hole. It is also not allowable, that other factors than those caused by the process substance to be measured effect the measuring results.
The present invention relates more particularly to a through hole construction in devices measuring the consistency, for example the percentage of solids in the mixture, of flowing solid-liquid-mixtures, e.g. paper stock and being of the type disclosed in U.S. Pat. No. 4,062,226. One of the requirements for devices of this type is that the resistance to torque about the transverse axis of the transfer element must be directly proportional to the actual torque applied by the sensing element to the transfer element. This kind of resistance or "counter-torque" results from the structure of the sealing member in the through hole. Due to the considerable friction and hysteresis caused by the internal friction or plasticity characteristic of elastomers used in the through hole construction, the above mentioned requirements are not sufficiently met.
In well-known measuring devices of the above type the leading-through of the transfer element is realized by disposing an elastic rubber seal, such as a O-ring, around the transfer member to lie against the side surfaces of the through hole. The leading-through is also realized by a bellows tube of metal fixed between the transfer element and the side of the hole. The above mentioned constructions are insufficient as far as the elasticity of the seal is concerned. When a rubber seal is used, the hysteresis error is a considerable drawback and, moreover, this error will change during the life-time of the seal. Further, the changes in the stiffness of the rubber during the life-time give rise to errors in the measurement. The elasticity of the rubber may also be temperature-dependent, and as a result of this in some cases the rubber seal must be heated for maintaining its temperature in a certain range. The adjustment of the measuring device to be independent of the changes of static pressure is also difficult. The drawback of the leading-through using the bellows tube is the possible clogging of the tube, which results in the stiffening of the through hole construction. This construction has further a considerable torsion stiffness, which causes extra tensions at the leading-through, thus making the measurement difficult.